1. Field of this Invention
This invention relates to double wall heat exchangers and methods of preparing such double wall heat exchangers.
2. Prior Art
Due to the possible toxicity of solar fluids, several codes of state and local governments have been enacted which require the heat exchanger tube coil to have two separate walls. The design of such double wall heat exchangers can be of two basic types, namely, vented and unvented. With the vented design, a failure of the inner coil will cause leakage at the terminal ends of the coil at a specified pressure (about 10 psig) between the tubes. With the unvented design, the terminal ends of the coil are sealed. The placing of one tube inside of another has been done in the past; however, in such cases, there is little or no metal contact surface between the tube walls resulting in poor heat transfer. The art has tried more elaborate schemes which have also been unsatisfactory.
U.S. Pat. No. 2,586,653 (Hill) produces a composite tube which has an outer tube with a helical outer fin. A matching internal helical groove is present on the outer tube. An inner tube has a helical outer rib that mates with the internal helical groove of the outer tube. There is no space between the inner tube and the outer tube (including the mating groove and rib) after they are formed. Hill forms the composite tube by using an outer tube that has a slightly larger inner diameter than the outer diameter of the inner tube. (A mandrel is usually inserted into the inner tube.) The mating rib of the inner tube is rolled up at the same time the material of the outer rib is extruded to form the mating fin. The mating rib of the inner tube is caused by the rolling pressure which formed the ribs of the outer tube. The outer tube is reduced in internal diameter and brought in complete contact with the inner tube.
In U.S. Pat. No. 3,750,444 (Bittner) in FIG. 3 shows an externally helically finned outer tube 1 and internally helically finned inner tube 3. The helical fins (ribs) have mating paths. The result is a helical passageway between the inner and outer tubes. The internal fins should cause quite a fluid flow pressure drop, etc.
FIG. 2 of U.S. Pat. No. 3,730,229 (D'Onofrio) shows an outer tube having internal helical grooves and an inner tube having mating internal helical grooves, the external protrusions of which fit in the helical grooves of the outer tube. Helical pathways are thereby formed between the inner and outer tubes. The inner helical tube is formed by twisting--see FIGS. 5 to 9. The internal helical groove of the outer tube is formed by deformation pressure when the internal helical tube is formed--see col. 4, lines 47 to 60. U.S. Pat. No. 4,111,402 (Barbini) shows two tubes, one inside of the other, which each have at least one spiral corrugation (fin) in opposite twist to the other. The spherical corrugations are each formed by the twist method. U.S. Pat. No. 2,913,009 (Kuthe) shrinks an outer tube around an inner helical tube. U.S. Pat. No. 2,724,979 (Cross) shows an inner tube inside of an outer helical tube.
U.S. Pat. No. 3,724,537 (Johnson) involves expanding an inner tube into the internal grooves of an outer finned tube by means of internal in-situ high pressure. The internal grooves of the outer tube are completely filled. U.S. Pat. No. 3,467,180 (Pensotti) shows an outer finned tube which has a series of internal longitudinal grooves. An inner tube is expanded into the longitudinal grooves. Pensotti also expands an inner tube having a series of external grooves against the smooth interior wall of the outer tube--a series of longitudinal passageways result. U.S. Pat. No. 4,031,602 (Cunningham et al.) teaches a method of making finned heat transfer tubes. U.S. Pat. Nos. 3,267,563 (Keyes I), 3,267,564 (Keyes II) show an internally finned tube telescoped in outer tube.
See also U.S. Pat. Nos. 3,887,004, 3,868,754, 3,878,593, 3,100,930, 3,267,563, 3,267,564, 2,693,026, 4,031,602, 1,970,481, 1,646,384 and 1,813,096.